Yb. Ma et Nc. Uren, TRANSFORMATIONS OF HEAVY-METALS ADDED TO SOIL - APPLICATION OF A NEW SEQUENTIAL EXTRACTION PROCEDURE, Geoderma, 84(1-3), 1998, pp. 157-168
A sequential extraction procedure, used to remove the heavy metals in
specifically adsorbed and easily reducible manganese (Mn) oxide fracti
ons, was used to study the transformation of heavy metals added to an
alkaline soil. Most of the endogenous Cu (86%) and Pb (79%) were found
in the residual fraction (RES) which was considered to he mainly high
ly crystalline Fe oxides and silicate minerals. The recently added Cu,
Pb and Cd existed mainly on the surfaces of the soil particles as rea
ctive fractions (water-soluble plus exchangeable and NaCaHEDTA-extract
able fractions) and as highly stable forms (RES fraction). There was a
particularly high affinity of Pb for Mn oxides. The concentrations of
metals in the reactive fractions were in the order: Cd > Cu > ph. Whe
n water-soluble heavy metals are added to the soil, they are rapidly r
etained by the soil. The reactive forms then slowly transform into hig
hly stable forms, The processes associated with the transformation of
added Cu and Pb can be described by a diffusion equation. The processe
s may be attributed mainly to the diffusion of the surface species int
o micropores and the entrapment in microporous solids. Unlike Cu and P
h, most of the exchangeable Cd transformed to the forms extracted with
NaCaHEDTA and to residual forms. The slow processes of the transforma
tion of Cd added to soil may be attributed to inner-sphere surface com
plexation via partial or complete dehydration of surface species. The
relative diffusion rate coefficients (D/r(2)) were found to be of the
order of 10(-10) to 10(-11) s(-1). Addition of CaCO3 decreased the rea
ctivity or extractability of added heavy metals through the increase i
n pH. (C) 1998 Elsevier Science B.V. All rights reserved.